To evaluate a stereoselective interaction for amiodarone and racemic warfarin, we performed a prospective study with its separated enantiomorphs. Single oral doses of racemic warfarin, 1.5 mg/kg, were administered to six normal subjects, with and without oral amiodarone, 400 mg daily, for the hypoprothrombinemic duration. Both the hypoprothrombinemia (P less than 0.001) and plasma warfarin concentrations (P less than 0.01) were significantly increased. The experiments were repeated separately with the R- and S-warfarin enantiomorphs. S-warfarin with amiodarone significantly increased both the hypoprothrombinemia (P less than 0.001) and plasma warfarin concentrations (P less than 0.01). R-warfarin with amiodarone significantly increased both the hypoprothrombinemia (P less than 0.001) and plasma warfarin concentrations (P less than 0.001). Thus amiodarone augmented the anticoagulant effect nonstereoselectively by reduced metabolic clearance of both warfarin enantiomorphs. Amiodarone and racemic warfarin can be a dangerous combination, particularly when either drug is added to a stabilized regimen of the other drug, unless the prothrombin times are monitored carefully.